Even if you accept that they will use the extra capacity and generate more revenue, that improves the per pound cost to fly the airplane. My wife and I just took a trip that included two flights costing $600+, and typical of 737 type flights. We weigh less than the saved weight on those brakes, so you could add at least one and maybe two more passengers. That additional revenue becomes significant over time, even if the percentage of the total revenue is small.
The bottom line is it costs fuel to carry weight. By being able to carry more passengers, you may be able to take one plane out of your fleet, or at least reduce the number of hours on each airplane by a small amount. Any way you calculate it, while the change is small, over time, it becomes a significant savings.